2.3 How make Processes a Makefile（make如何处理makefile文件）
By default, make starts with the first target (not targets whose names start with ‘.’（以.开头的不是目标）). This is called the default goal（最终目标）. (Goals are the targets that make strives（努力） ultimately（最终） to update. You can override this behavior using the command line or with the .DEFAULT_GOAL special variable .
In the simple example of the previous section, the default goal is to update the executable program edit; therefore, we put that rule first.
Thus, when you give the command:
makemake reads the makefile in the current directory and begins by processing the first rule（make从第一条规则开始执行）. In the example, this rule is for relinking edit; but before make can fully（完全地） process this rule, it must process the rules for the files that edit depends on, which in this case are the object files. Each of these files is processed according to its own rule. These rules say to update each ‘.o’ file by compiling its source file. The recompilation（编译） must be done if the source file, or any of the header files named as prerequisites, is more recent（新的） than the object file, or if the object file does not exist.
The other rules are processed because their targets appear as prerequisites of the goal. If some other rule is not depended on by the goal (or anything it depends on, etc.), that rule is not processed, unless you tell make to do so (with a command such as make clean).
Before recompiling an object file, make considers updating its prerequisites, the source file and header files. This makefile does not specify anything to be done for them—the ‘.c’ and ‘.h’ files are not the targets of any rules—so make does nothing for these files. But make would update automatically generated C programs, such as those made by Bison or Yacc, by their own rules at this time.
After recompiling whichever object files need it, make decides whether to relink edit. This must be done if the file edit does not exist, or if any of the object files are newer than it. If an object file was just recompiled, it is now newer than edit, so edit is relinked.
Thus, if we change the file insert.c and run make, make will compile that file to update insert.o, and then link edit. If we change the file command.h and run make, make will recompile the object files kbd.o, command.o and files.o and then link the file edit.